The present invention relates to a method and a device for calibrating a position sensor which is placed on the shaft of a permanent magnet synchronous motor in order to control the position of its rotor relative to a magnetic field produced by a stator of the permanent magnet synchronous motor.
Electric machines are key automotive components. They may be utilized as starting motors, as alternators that generate electricity from engine motion and as electric drive motors in modern hybrid electric vehicles. A permanent magnet synchronous motor (PMSM) is suited, amongst others, for use as an electric drive motor for a hybrid electric vehicle, owing to a number of advantageous attributes of a PMSM, such as high reliability and high efficiency.
The efficiency of a PMSM is highly dependent upon the position of the rotor of the PMSM relative to the magnetic field produced by the stator windings. Therefore various methods have been developed to control the relative position of the rotor and the magnetic field. In particular control methods are known that use a position sensor for detecting and controlling the position of the rotor relative to the magnetic field.
When a position sensor is used for controlling a PMSM, serving for instance as a power engine for a vehicle, a position sensor calibration procedure must usually be carried out after position sensor assembly (end of line approach) and/or with every start-up and shut-down of the vehicle. This procedure is necessary to ensure motor efficiency and to produce the commanded torque with the highest possible accuracy.
Inaccurate calibration of the position sensor can cause torque accuracy problems and reduced motor efficiency. Also unpredictable power train behavior may occur with inaccurate position sensor information.
In general position sensor calibration methods can be divided into DC current based approaches and AC current based approaches. Both approaches come with some distinct advantages and disadvantages. AC current based approaches usually do not require any PMSM rotor movement, in contrast to DC current based approaches. On the other hand, DC current based approaches usually do not cause audible noise while AC current based approaches in most cases result in at least some audible noise.
Some known DC current based methods use the fundamental electromagnetic behavior of a PMSM; that is, in a magnetic field of the stator there is a torque-free position of the rotor in which the magnetic field does not exert a torque on the rotor. However, a torque will be produced when the rotor position deviates from the torque-free position. This effect is used by DC current based methods which let the rotor move to its torque-free position in a static magnetic field generated by a DC current supplied to the stator windings of the PMSM. The supplied DC current thus fixes the torque-free rotor position, and the position measured by the position sensor when the rotor has taken its torque-free position provides an angle offset between the rotor position and the position measured by the position sensor that can be used to calibrate the position sensor.
However, such methods can cause rotor speed transients which are undesired and can excite secondary effects in a mechanical system to which the PMSM is connected. In particular this may result in overshoot and variable settling times.
U.S. Pat. No. 5,796,228 discloses a method of controlling a rotary magnet multi-phase synchronous motor and a control system based on this method using a position detector for detecting a magnet position of the synchronous motor and a drive unit for driving the synchronous motor. After the synchronous motor reaches a constant speed, current and drive voltage of the synchronous motor are measured to calculate a phase correction value between a biaxial coordinate axis based upon the installation error of the position detector and a biaxial coordinate system from conversion of a polyphase AC coordinate system based on the measured current and drive voltage, constants of the synchronous motor, and the constant speed. The rotor position of the synchronous motor is corrected based on this phase correction value.
One object of the invention is to provide an improved method and apparatus for calibrating a position sensor which is placed on the shaft of a PMSM in order to control the position of a rotor of the PMSM relative to a magnetic field produced by a stator of the PMSM.
This and other objects and advantages are achieved by the method according to the present invention, for calibrating a position sensor placed on a shaft of a PMSM in order to control the position of a rotor of the PM relative to a magnetic field produced by a stator of the PM. According to the invention, a DC test current is supplied to stator windings of the PMSM to generate a definite magnetic field, and a motor speed caused by the injected DC test current is then adjusted to zero by modifying the rotor position. The position measured by the position sensor at zero motor speed relative to the magnetic field is used to calibrate the position sensor.
This method is thus a DC current based calibration method which makes use of the torque-free rotor position in the magnetic field generated by the DC test current, as known DC current based calibration methods outlined above do as well. However, in contrast to these known methods, the rotor does not move freely to its torque-free position. Rather it is moved to the torque-free position in a controlled and smooth manner by adjusting the motor speed to zero through a corresponding active modification of the rotor position. The method according to the invention thus has all of the advantages of known DC based calibration methods while avoiding some specific deficiencies of these known methods, such as position overshooting and undesired rotor oscillations about the torque-free position.
In an advantageous embodiment of the method the motor speed is determined from signals of the position sensor. To this end it is particularly advantageous that the signals of the position sensor are differentiated with respect to time and that the motor speed is determined as an angular velocity of the rotor from the differentiated signals of the position sensor.
Using signals of the position sensor to determine the motor speed has the advantage to reduce the number of components to control the PMSM, as no sensor other than the position sensor is needed to determine the motor speed. The determination of the motor speed as an angular velocity of the rotor from the differentiated signals of the position sensor has the additional advantage that an angular velocity can be easily measured and does not depend on the dimensions of the PMSM and its components.
Furthermore, preferably the strength of the DC test current is kept constant during the adjustment of the motor speed.
This fixes the magnetic field of the stator and thus the direction of the torque-free rotor position used as a reference to calibrate the position sensor and thereby advantageously simplifies the calibration of the position sensor because variations of the strength and/or direction of the magnetic field need not to be taken into account.
A device according to the invention for calibrating a position sensor placed on a shaft of a PMSM in order to control the position of a rotor of the PM relative to a magnetic field produced by a stator of the PM comprises speed detecting means to detect a current motor speed and speed controlling means to adjust the motor speed to zero by modifying the rotor position while a DC test current is supplied to stator windings of the PMSM.
In a preferred embodiment of the device the position sensor is connected to the speed detecting means and the speed detecting means is connected to the speed controlling means.
In this way signals of the position sensor can be directly supplied to the speed detecting means and signals of the speed detecting means can be directly supplied to the speed controlling means, thereby advantageously simplifying the calibration of the position sensor.
Preferably, the speed detecting means comprise a differentiator for determining a time derivative of signals of the position sensor.
By means of such a differentiator a motor speed can be determined as an angular velocity from signals of the position sensor.
Furthermore, preferably the speed controlling means comprise a proportional-integral offset-controller to modify the rotor position depending on the detected motor speed.
The use of a proportional-integral controller to adjust the motor speed to zero advantageously suppresses position sensor signal noise and thus stabilizes the adjustment of the motor speed against such noise.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.